Gelatin is prepared by denaturation of collagen. Collagen is a triple stranded helix protein found in skin, cartilage and bones of humans and animals where it serves as a structural component in the extracellular matrices (ECM). The strands found in collagen are in gelatin found as mixtures together with oligomers, breakdowns and other polypeptides forming small local collagen-like triple-helical areas.
The typical sources of gelatin for industrial applications are pigs, cows and fish recovered from collagen by hydrolysis. There are several varieties of gelatin, the composition of which depends on the source of collagen and the hydrolytic treatment used. Type A gelatin results from an acid hydrolysis of typically skin from pigs whereas type B gelatin results from alkaline hydrolysis of cattle hides and bones.
Gelatin has sol gel properties, which are thermo reversible. Above about 37° C. gelatin is in the sol state, whereas below about 37° C. gelatin is in the gel state. The quality of gelatins is commonly characterized by bloom, e.g. according to AOAC standards gelatin bloom test and BS757.
Gelatin is widely applied in pharmaceuticals, foods, medical dressings and technical applications e.g. photographic paper. Due to poor fiber forming properties of gelatin, there are few reports of fibers and non-woven made of pure gelatin and primarily made by electro-spinning using organic solutions (Huang Z M et al. (2004), Polymer 45, 5361-5368)(Zhang et al. (2004) J Biomed Mater Res Part B: Appl Biomater 72B: 156-165). One recent report describes electrospinning of gelatin dissolved solely in water (Li J. et al., Biomacromolecules 2006, 7, 2243-2247). Most often the fiber forming properties of gelatin are improved by addition of another polymer or by grafting or substituting chemical groups to the gelatin chain. Of other examples coating of fibers with gelatin can be mentioned (Lin FH et al. (2000) Materials Chemistry and Physics 64, 1889-195) (WO03087444A).
The production of fibers from protein solutions has typically relied upon the use of wet or dry spinning processes (Martin et al. Processing and Characterization of Protein Polymers; McGrath, K. and Kaplan, D., Ed.; Birkhauser: Boston, 1997, pp. 339-370; Hudson, S. M. The Spinning of Silk-like Proteins into Fibers; McGrath, K. and Kaplan, D., Ed.: Birkhauser: Boston, 1997, pp. 313-337).
Wet spinning, more commonly used, involves the extrusion of a protein solution through a spinneret into an acid-salt coagulating bath, which usually contains aqueous ammonium sulfate, acetic acid, isopropanol, or acetone (Nagura et al. (2002) Polymer Journal, Vol 34, No 10, 761-766), (JP2001089929), (Fukae R et al. (2005) Polymer 46, 11193-11194). Alternatively, dry spinning consists of extrusion into an evaporative atmosphere. Both approaches yield large diameter fibers, which do not mimic the morphological characteristics of native collagen fibers. Furthermore, both strategies rely on biologically toxic solvent systems that preclude the fabrication in real time of hybrid protein-cell constructs.
By the electro-spinning process it is possible to make fibers of pure gelatin. E.g. US 2004/0110439 describes durable, load bearing prosthetic materials of cross-linked elastin, cross-linked elastin mimetic protein, cross-linked collagen and/or cross-linked gelatin. Fibers with a diameter in the 200-3,000 nm range are electrospun and a non-woven created.
However the electro-spinning process has low output and relies on the use of expensive and harmful solvents. In wound healing application low density products of gelatin is often made by freeze drying, which has the disadvantage of being a costly process and a batch process.
U.S. Pat. No. 5,017,324 describes the formation of fibers with particles by using one or more spray guns intermixing powder, particulate or strand-like material with the fibrous material to form a non-woven pad.